The origins of today’s mass extinction

The extinction symbol is a spare graphic that began to appear on London walls and sidewalks a couple of years ago. It has since become popular enough as an emblem of protest that people display it at environmental rallies. Others tattoo it on their arms. The symbol consists of two triangles inscribed within a circle, like so:

“The triangles represent an hourglass; the circle represents Earth; the symbol as a whole represents, according to a popular Twitter feed devoted to its dissemination (@extinctsymbol, 19.2K followers), “the rapidly accelerating collapse of global biodiversity” — what scientists refer to alternately as the Holocene extinction, the Anthropocene extinction, and (with somewhat more circumspection) the sixth mass extinction.

Collages of dodo skull and human skull by David McLimans

It’s always been a difficult planet. About 4 billion species have emerged since the place became habitable, 3.5 billion years ago. Ninety-nine percent of those species no longer exist. But five times in prehistory things went truly pear-shaped for the living. All schoolchildren know about one of these events, not only because it wiped out almost every dinosaur unlucky enough to be wingless but also because it did so by way of a flaming rock the size of Staten Island, which slammed into the Yucatán Peninsula at a speed of about 45,000 miles per hour. The impact vaporized everything in what is now North America, caused continent-reshaping tsunamis, released enough dust into the atmosphere to block out the light of the sun, and turned the oceans into a sulfuric stew for millions of years.

This was the most dramatic of what even the hardest-nosed academic journals tag as the Big Five. It wasn’t the worst. That distinction goes to the so-called end-Permian event, which took place about 252 million years ago. No one knows exactly what happened then, other than that the level of carbon dioxide in the atmosphere spiked, pushing up global temperatures and upending the chemical composition of the oceans. Warmer oceans may also have led to the flourishing of bacteria that produce hydrogen sulfide, a substance that, to put it mildly, most life-forms don’t find hospitable. Ninety-six percent of all species disappeared. The end-Permian event has been called the mother of mass extinctions. It has also been called the Great Dying.

These are justifiably sensational terms — but they perhaps give a mistaken impression of the speed with which mass extinctions take place. A giant asteroid is exceptional: the dinosaurs may have died out in a matter of months. The other events happened on a geologic timescale, grinding away slowly at the planet. The end-Permian played out over more than 100,000 years. The extinction event that wiped out the trilobites, at the end of the Ordovician period, and the one that wiped out the giant reptiles, at the end of the Triassic, played out over millions.

For most observers, scientific and otherwise, speed is a defining feature of our present-day extinction crisis, which is happening at such a rate that we — a species with a current average life span of sixty-seven years — are able to notice it happening. All across the planet, in every ecosystem, on every landmass and in every body of water, plants, animals, and microorganisms are now dying out within decades rather than millennia. The International Union for Conservation of Nature, a Switzerland-based organization that monitors the conservation status of more than 71,000 species, lists nearly 30 percent of them as threatened. “It is estimated,” writes Elizabeth Kolbert in The Sixth Extinction, her thorough and fascinating new book, “that one-third of all reef-building corals, a third of all freshwater mollusks, a third of all sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion.” The widely agreed-upon benchmark for a mass-extinction event is a loss of 75 percent of the planet’s biomass. We’re not there yet, but we’re on our way. According to Anthony Barnosky, a geologist at the University of California, Berkeley, if the IUCN’s assessment can be taken as a guide to the future — and few doubt that it can, unless extraordinary political and economic measures are taken — the Big Five could become the Big Six by as early as 2400 a.d.

What does this planetary denuding look like in real time? The extinction symbol’s Twitter feed gives us some hint.

Honeybee decline decisively linked to pesticide useDugong population in Indonesia being decimated by industrial pollutionGorillas and chimps are threatened by human disease.Overfishing causes Pacific bluefin tuna numbers to drop 96%Iceland to kill endangered fin whales for dog food.West Africa’s last rainforest, an area 50% bigger than Wales has been sold off to logging companies.Plankton heading for extinction due to climate change, and this could trigger the collapse of entire ecosystemsChina’s Last Wild Indochinese Tiger Killed and Eaten By A Villager

And so it goes, 3,600 dispatches and counting from an ailing biosphere. To read the feed from beginning to end is a stupefying experience, like going through the records from some vast terminal ward. But as with almost all nontechnical writing on extinction, that isn’t the spirit in which the feed is intended. The purpose isn’t documentary. This is a cri de coeur — an indictment for mass murder against a very familiar defendant. A post on April 22, 2012, encapsulates the tone of the project by way of a quotation attributed to Albert Schweitzer and used by Rachel Carson in her dedication to Silent Spring: “Man has lost the capacity to foresee and to forestall. He will end by destroying the earth.”

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